The present invention relates to a capacitance type accelerometer using a semiconductor and more particularly to an electro-static capacitance type accelerometer suitable for body control or engine control of an automobile.
It is required for an accelerometer for an automobile to detect acceleration of relative-low level (0 through .+-.1.0G) and the low frequency (0 to 10 Hz) at high accuracy. In addition, 1G=9.8 m/S.sup.2 herein.
Such accelerometer, as well-known, includes a piezoelectric sensor conventionally utilizing a piezoelectric effect of a piezoelectric material, an electromagnetic servo system sensor having an electromagnetic feedback mechanism, a magnetic sensor utilizing a differential transformer, an optical sensor utilizing a photo-interrupter, and a strain gauge system or a capacitance system sensor utilizing a fine processing technique of silicon. Among them the capacitance type accelerometer utilizing the fine processing technique of silicon is considered the most favorable, which can detect acceleration of the low frequency at high accuracy and provide an inexpensive sensor.
The largest problem for packaging the capacitance type accelerometer is the package of the lead for the electrode. Therefore, several methods are known.
A first example, as disclosed in U.S. Pat. No. 4,483,194, is a method to fetch the lead for the movable electrode or the fixed electrode through a P.sup.+ diffusion region. The method has the problem in that etching is complicated and yield when bonded is poor, the P.sup.+ diffusion region becomes a large stray capacitance, and its voltage dependency is large, thereby deteriorating the detection system.
A second example, as shown in Pages 336 to 339, Transducers '87 (the 4th International Conference on Solid-State Sensors and Actuators), is a method to fetch the lead from a gap at the bonding portion of the silicon plate and a glass base. The method, when a detected chip is diced by a usual dicing saw after the silicon plate and glass base are bonded in the wafer state, water content or chips enters into a space around the electrode through the gap, thereby creating the problem in that the performance as the accelerometer is not obtainable.
A third example, as disclosed "Miniature Silicon capacitance absolute pressure sensor", Page 255 to 260, I Mech E, 1981, is a method to fetch the lead through a bore perforating a glass base. This example is applied to the pressure sensor, in which the problem is created in the poor lead connection at one bore end caused by variation in metallizing condition and the temperature characteristic deterioration caused by a large difference in coefficient of thermal expansion between the solder charged in the other bore end and the glass base.
A fourth example, as shown in U.S. Pat. No. 4,609,968, is a method of using a silicon plate to the fixed electrode to package the silicon plate itself as part of the lead. In this example, a silicon plate to be the fixed electrode is applied at the periphery thereof with glass powder and baked and then polished at the surface to be like a mirror. A movable electrode and a silicon plate having a beam for supporting the electrode are laminated and bonded with anodic-bonding. The anodic bonding is to apply high voltage at a high temperature to glass and silicon to be bonded, in which the movable electrode is displaced by an electrostatic force and contacts with the silicon plate of fixed electrode so as to be discharged, whereby both the members cannot be bonded. In order to avoid this, a gap between the movable electrode and the fixed electrode must be previously designed to be larger, or a spring constant of the beam must be designed to be larger (in brief, deformation of the movable electrode with respect to the unit acceleration). As the result, the problem is created in that high sensitivity essentially possessed by the capacitance type accelerometer must largely be sacrificed.
The above-mentioned conventional technique is not considered as to the electrode lead package method, thereby creating the problem in the productivity, detection accuracy or sensitivity.